1. Field of the Invention
This invention relates to a polishing method and a polishing apparatus which use an abrading plate (fixed abrasive polishing tool). More particularly, the invention relates to a method and an apparatus for polishing an object to be polished, such as a semiconductor wafer, in a flat and mirror-like state.
2. Description of the Related Art
In recent years, with the increased integration of semiconductor devices, interconnection of circuits has become miniscule, and the devices to be integrated have been miniaturized. This may require the step of removing a film, formed on the surface of a semiconductor wafer, by polishing to flatten the surface. As means of this surface-flattening or planarization, polishing by means of a chemical and mechanical polishing (CMP) device is performed. This type of CMP device has a turntable pasted with a polishing cloth (pad), and a top ring. An object to be polished is interposed between the turntable and the top ring. With the turntable being kept under a constant pressure from the top ring, and the polishing cloth being supplied with a polishing solution (slurry), the turntable and the top ring rotate to polish the surface of the object to be polished into a flat, mirror-like form.
The above-described CMP process using a polishing solution (slurry) performs polishing while supplying a relatively soft polishing cloth with the polishing solution (slurry) containing a large amount of abrasive particles. Thus, this process is problematic in terms of pattern dependency. Pattern dependency refers to the fact that an irregular pattern on the semiconductor wafer, which has existed before polishing, results in the formation of mild irregularities after polishing, thus making it difficult to obtain perfect flatness. That is, irregularities with small pitches lead to a high polishing speed, while irregularities with large pitches lead to a low polishing speed, with the result that the fast polishing portions and the slow polishing portions together result in the formation of the mild irregularities. Besides, the polishing process using the polishing cloth (pad) polishes both the projections and the depressions of the irregularities, thus posing difficulty in achieving a so-called self-stop function, i.e., the function that only the projections are polished away to bring complete flatness, and at this time, polishing stops.
On the other hand, study is under way about polishing of a semiconductor wafer using so-called fixed abrasive particles (an abrading plate), i.e., abrasive particles, such as cerium oxide (CeO2), fixed with the use of a binder, such as phenol resin. Polishing with such an abrading plate is advantageous in the following respects: The abrading plate is hard unlike that in conventional CMP. Thus, the projections of the irregularities are polished preferentially, while the depressions of the irregularities are polished with difficulty, so that absolute flatness is easy to obtain. Depending on the composition of the abrading plate, moreover, the self-stop function emerges in the following manner: Polishing of the projections is completed to impart a flat surface. At this time, the polishing speed markedly lowers, and polishing actually does not proceed any more. In addition, polishing with an abrading plate does not use a polishing solution (slurry) containing a large amount of abrasive particles, thus conferring the advantage that the burden of an environmental problem is lessened.
Polishing using an abrading plate, however, poses the following problems: If the composition of the abrading plate is such that the binding force of a binder for binding abrasive particles is high, the abrasive particles minimally exhibit a self-generating effect during polishing. Immediately after dressing, a relatively high polishing speed is obtained. However, as polishing proceeds, the polishing speed decreases, failing to obtain a sufficient polishing speed. In the case of an abrading plate with a low binding force of a binder for binding abrasive particles, the abrading plate is brittle as a whole. Since the abrasive particles easily self-generate, a relatively high polishing speed is obtained. However, not only the projections, but the depressions of the irregularities of the surface of workpiece are also polished. After polishing, a completely flat surface is difficult to obtain, arousing a problem with so-called step characteristics. In addition, such an abrading plate minimally achieves the self-stop function by which progress of polishing automatically stops after polishing of only the projections is completed.
Hence, an abrading plate showing the self-stop function is in a relatively narrow range in which the proportions of a binder, abrasive particles, and pores are well balanced. Such an abrading plate does not necessarily provide the desired polishing speed stability and step characteristics for the object to be polished. Materials to be polished range widely, including silicon substrates, polysilicon films, oxide films, nitride films, and interconnection layers comprising aluminum or copper materials. Producing abrading plates, which have stability of polishing speed, satisfactory step characteristics, and self-stop function in response to these various objects to be polished, has been very difficult.
It may be desired to enclose a third substance in an abrading plate in order to decrease scratches or promote the reaction. Enclosure of such a substance, however, changes the composition conditions, and results in the failure to exhibit the self-stop function.
In addition, the polishing speed during polishing of a semiconductor wafer by use of an abrading plate is high immediately after dressing. However, the polishing speed gradually decreases, so that the polishing speed is not stable. To stabilize the polishing speed, dressing needs to be performed before each polishing. Dressing before each polishing requires a certain period of time. Thus, a throughput declines in practical use, and its decline lowers productivity.
The present invention has been accomplished in light of the foregoing circumstances. Its object is to provide a method and apparatus for polishing a substrate, which can always exhibit the self-stop function, without being restricted by the composition of the fixed abrasive, and without being restricted by the type of the substrate to be polished.
A first aspect of the invention is a method for polishing a device wafer by use of a fixed abrasive, the device wafer having projections and depressions formed on a surface thereof. The method comprises polishing the device wafer while supplying a surface active agent and/or while performing dressing of a surface of the fixed abrasive.
When polishing is performed while a surface active agent is being supplied, polishing of a blanket wafer (a wafer with a flat surface without irregularities) is known to proceed minimally. That is, the supply of a surface active agent enables the self-stop function to be exhibited. To carry out polishing while dressing the fixed abrasive surface, moreover, causes a large amount of free abrasive particles to be always self-generating during polishing, thus stabilizing the polishing speed. To perform polishing, while supplying a surface active agent and performing dressing, therefore, can present many free abrasive particles, obtain a relatively high polishing speed, and show the self-stop function. Accordingly, the self-stop function can be shown in a wide range of compositions for a fixed abrasive, without restriction by the conventional composition conditions for the fixed abrasive.
The method for polishing a device wafer may comprise continuing polishing while starting supply of the surface active agent, before polishing of the device wafer proceeds to flatten the projections. According to this feature, supply of the surface active agent is started before polishing of the device wafer proceeds to flatten the projections, for example, when 2T/3 has passed, provided that the time required until flattening is T. By this measure, the self-stop function can be exhibited efficiently. That is, supply of the surface active agent is not performed at an initial stage, but rather at an intermediate stage of polishing. The amount of the surface active agent to be otherwise used in these stages can be saved, whereby the cost for polishing can be decreased.
The method for polishing a device wafer may comprise polishing the device wafer while dressing the fixed abrasive, and stopping dressing and continuing only polishing, before polishing of the device wafer proceeds to flatten the projections. According to this feature, dressing is stopped before the projections of the device wafer are flattened, for example, when 2T/3 has passed, provided that the time required until flattening is T. By this measure, the amount of free abrasive particles exhibiting a self-generating effect can be decreased. Thus, the polishing speed lowers, and the self-stop function can be exhibited. Therefore, the range in which the self-stop function appears can be widened by polishing with a fixed abrasive, without the use of chemicals, such as surface active agents.
The method for polishing a device wafer may comprise performing polishing while supplying a chemical solution contributing to promotion of a reaction. According to this feature, it becomes possible to increase the polishing speed without using dressing, and stabilize the polishing speed.
The method for polishing a device wafer may comprise polishing the device wafer by use of the fixed abrasive, and then performing touch-up polishing of the device wafer. According to this feature, abrasive particles deposited on the surface of the wafer, and flaws (scratches) on the wafer surface can be removed.
A second aspect of the invention is an apparatus for polishing a device wafer by use of a fixed abrasive, the device wafer having projections and depressions formed on a surface thereof. The apparatus comprises means for promoting polishing, and means for suppressing polishing.
The means for promoting polishing is preferably dressing means for performing dressing during polishing, or means for supplying a chemical solution for promoting polishing. The means for suppressing polishing is preferably means for supplying a surface active agent.
The size of the wafer, which is an object to be polished, and the size of the table-shaped fixed abrasive may be in the following relationship
2Rw greater than Rf greater than Rw/2
where Rw is the radius of the wafer, and Rf is the radius of the table-shaped fixed abrasive. And, the wafer rotates on its own axis, and the fixed abrasive rotates on its own axis.
According to these features, some variation in the relative speed occurs in the polished surface of the device wafer to be polished, relative to the polishing surface of the fixed abrasive. Because of the above-described self-stop function, however, once the projections are flattened, progress of polishing stops. As a result, a uniform flat surface is obtained. Thus, the diameter of the table-shaped fixed abrasive relative to the diameter of the wafer can be reduced compared with earlier technologies. Consequently, the apparatus and the fixed abrasive can be made compact and economical, without deterioration of polishing performance.
A third aspect of the invention is an apparatus for polishing a device wafer by use of a fixed abrasive, wherein the device wafer has projections and depressions formed on a surface thereof. The apparatus comprises: means for promoting polishing; and means for suppressing polishing: and wherein a size of the wafer, which is an object to be polished, and a size of the table-shaped fixed abrasive are in the following relationship
2Rw greater than Rf greater than Rw/2
where Rw is a radius of the wafer, and Rf is a radius of the table-shaped fixed abrasive. The radius Rf of the table-shaped fixed abrasive is greater than a distance between a center of the wafer and a center of the table-shaped fixed abrasive. The wafer rotates on its own axis, and the fixed abrasive rotates on its own axis.
According to these features, some variation in the relative speed occurs in the polished surface of the substrate to be polished, corresponding to the polishing surface of the fixed abrasive. Because of the above-described self-stop function, however, once the projections are flattened, progress of polishing stops. Even if the polished surface of the substrate to be polished leaves the polishing surface of the fixed abrasive, the inclination of the top ring is suppressed, because the center of gravity of the wafer always lies on the table-shaped fixed abrasive. As a result, a uniform flat surface can be obtained. Thus, the diameter of the table-shaped fixed abrasive relative to the diameter of the wafer can be reduced compared with earlier technologies. Consequently, the apparatus and the fixed abrasive can be made compact and economical without deterioration of polishing performance.
The apparatus for polishing a device wafer may further comprise touch-up devices for performing touch-up polishing of the device wafer for which polishing has been performed by use of the fixed abrasive. This feature makes it possible to remove abrasive particles deposited on the surface of the wafer, and flaws (scratches) on the wafer surface caused by fixed abrasive polishing.